I use one on my lower legs and don't feel anything when using it. When I bought it, many years ago now, it had a plastic 'ice pack' that clipped onto it, the idea being that the v cold plastic rolled over your leg just ahead of the epilator to numb the area. I found I didn't need it but, if they still do these, it might be worth a try for you.
I have to say I've never used it on underarms, I stick to shaving there, and never had the need to go above knee so no idea how it would feel on the softer (ie flabbier 🤭) skin there.
I think as we're all so different by way of pain threshold you'd probably have to take the plunge and invest in one to see if it's for you.
Suzie